1. Field of the Invention
The present invention relates to a biaxially oriented film having at least one layer where at least one layer of the biaxially oriented film is composed virtually exclusively of polyethylene naphthalate bibenzoate (PENBB).
2. Description of Related Art
In the field of carrier films for magnetic recording materials, such as video tapes, there is a continuous need to accommodate more data in a smaller space. To this end it is necessary, on the one hand to reduce the thickness of the carrier film even further, and thus to increase the tape length for a given cassette volume. On the other hand, it is necessary to use new coatings having a higher write density, such as ME (=metal evaporated) coatings or "perpendicular recording", the application technique for which signifies exposure of the carrier film to higher temperatures.
The same demands for smaller film thicknesses and increased temperature stability also are made in the case of capacitor dielectrics. This offers advantages with respect to the space utilization of the capacitor and in the soldering operation.
It is possible to reduce the thickness of the carrier film, however, only if the film has an improved specific mechanical stability (modulus of elasticity) both in the longitudinal and in the transverse directions for a constant thickness. Conventional carrier films made of polyethylene terephthalate (PET) reach the limit of mechanical stability at a thickness of about 7 .mu.m. Recently, attempts have been made to replace PET carrier films by films made of polyethylene naphthalate (PEN). It is true that, compared with PET, PEN has a higher mechanical stability (in the case of "balanced" films, for example, modulus of elasticity values of up to about 8 GPa for PEN when composed to values of about 6 GPa PET films), but the thermal stability of the film is only slightly higher. Films can be produced from polyphenylene sulfide (PPS) which, although they have a higher stability to heat than PEN films, the PPS films have a mechanical stability which is lower than PEN films, and even somewhat lower than that of a PET film.
There have been numerous attempts to replace PET by copolyesters for use as fiber materials. Thus, a number of publications describe copolyesters made of ethylene terephthalate/naphthalate or ethylene terephthalate/-bibenzoate (for example JP 54/095634 (Toyobo), JP 02/038422 (Toyobo), JP 01/113811 (Teijin), EP-A-0 202 631 (Teijin). In JP 03/252449 (Teijin), copolyesters are described in which the dicarboxylic acid component is selected from, Inter alia, terephthalic acid, naphthalene-2,6-dicarboxylic acid and 4,4'-dicarboxydiphenyl and the diol component is derived from ethylene glycol and resorcinol. Because of their terephthalate content, which is still fairly high, films made from these materials cannot provide any substantial improvement in mechanical stability and thermal stability. In JP 50/135333 (Teijin), fiber materials are described which are made of a copelyester of polyethylene naphthalate with 0.5 to 20% bibenzoate (derived from 4,4'-dicarb-oxydiphenyl). According to the teaching of this document, a bibenzoate content higher than 20% is disadvantageous because the melting point of the resulting polymer is too low.
U.S. Pat. No. 3,008,934 describes fiber- and film-forming copelyester compositions, which contain 4,4'-bibenzoic acid as acid-derived components. The publication does not mention biaxially oriented films produced from these materials. In light of the present expert knowledge in this field, it is indeed very unlikely that a person skilled in the art would have used the copolyesters described therein for producing biaxially oriented films. U.S. Pat. No. 3,008,934 states that pure polyethylene-4,4'-bibenzoate has an extraordinarily high crystallization rate. This fact would have led the skilled artisan away from producing biaxially oriented films containing bibenzoate, especially in the high amounts taught therein (40-90% by weight) because a premature crystallization leads to rapid embrittlement of the film even during the production process. Thus, premature crystallization leads to film tears even before biaxial orientation is achieved.
Fibers comprised of PENBB with a bibenzoate content of 0.5 to 20% are disclosed in JP 50/135333 (Teijin). There was skepticism in the art that films, in particular biaxially oriented films, could be prepared from these compositions for the same reasons as described above (premature crystallization). In this regard, the requirements for fiber production are less stringent since a uniaxial fiber orientation can be carried out much faster than a biaxial film orientation.
The fact that biaxially oriented films having improved mechanical and thermal properties can be prepared at all with bibenzoate contents above 20% is surprising in light of the teachings. of JP 50/135333 (melting point lowered too greatly with bibenzoate content of above 20%) and U.S. Pat. No. 3,008,934.
Fibers made from PENBB polymers are described in U.S. application Ser. No. 07/735,553 abandoned. This application also proposes the production of films from these polymers, but contains no disclosure with respect to the thickness of the film. There is a need in the art to produce thin films for use in magnetic tapes and capacitor dielectrics.
Thus there continues to be a need for films which are more rigid and more resistant to heat especially for the production of thinner magnetic tapes and capacitor dielectrics.